Titanium or titanium alloys having an anodized surface layer and method of forming

ABSTRACT

1. A METHOD FOR OBTAINING TITANIUM OR TITANIUM ALLOY PARTS PROVIDED WITH A LOW FRICTION COEFFICIENT SURFACE AND CAPABLE OF RESISTING GRIPPING OR SEIZURE BY RUBBING UNDER HIGH PRESSURE WHICH CONSISTS OF FORMING ON THE SURFACE OF THE PARTS AV POROUS AND RESISTANT OXIDE LAYER HAVING A THICKNSSS OF MORE THAN 1 MICRON AND AN OPEN POROSITY RANGING BETWEEN 20% AND 80% WITH DIMENSIONS OF PORE SECTIONS RANGING BETWEEN 2 AND 5 SQUARE MICRONS, SAID OXIDE LAYER BEING FORMED BY ANODIZATION UNDER DC IN AN AQUEOUS SOLUTION WHICH CONTAINS A QUANTITY OF SULFURIC ACID RANGING BETWEEN 6% AND 35% BY WEIGHT AND A QUANTITY OF HYDROCHLORIC ACID RANGING BETWEEN 0.7% AND 8.5% BY WEIGHT.   D R A W I N G

Oct. 8, 1974 J A. CHEVALlE-R lrrAL 3,840,442

TITANIUM OR iITAHIUM ALLOYS HAVING AN AKODIZED SURFACE LAYER AND METHOD OF FORIING Filed April 7. 1972 Fig. 1

Fig. 2

United States Patent Olhce 3,840,442 Patented Oct. 8, 1974 U.S. Cl. 204-35 N Claims ABSTRACT OF THE DISCLOSURE The invention relates to a process for forming a porous, resistant layer of titanium oxide by anodization on titanium and titanium-based alloys, and is distinguished by the fact that anodization is carried out in an aqueous solution containing from 6% to 35% by weight of sulphuric acid and from 0.7% to 8.5% by weight of hydrochloric acid and, optionally, from to 200 g./l. of perchloric acid and/or from 20 to 150 g./l. of nitric acid.

This invention relates to a process for the formation by electro-chemical oxidation of a thick, porous and mechanically resistant layer of titanium oxide in the surface of articles of titanium or titanium-based alloys.

A layer of this kind, the thickness of which can vary from 1 micron to several tens of microns, depending upon the duration of anodization, enables the surface of the anodized metal to be effectively lubricated through the penetration of lubricants into the pores of the anodized layer. It increases the resistance to wear and the resistance to corrosion of the surface of the metal.

Titanium and titanium-based alloys have acquired particular interest in industry and, in particular, in the aero nautical industry, by virtue of their remarkable mechanical properties and their low specific gravity of only 45. Unfortunately, they have a marked tendency towards seizure when in moving contact with conventional materials and, in particular, steels. The provision of a lubricant between the surfaces in contact does not have a lasting effect because the lubricant is not retained by the titanium surface.

To overcome this disadvantage, the surface of titanium articles has been subjected to various treatments, including the deposition of metals or ceramic compounds either by electrolysis, by spraying or by vacuum deposition, the diffusion of interstitial atoms, chemical or electrochemical treatments.

The treatment of titanium and its alloys by anodic oxidation involves considerable difiiculties. Baths recommended for the anodization of titanium include aqueous solutions of sulphuric acid with a concentration of 150 to 180 g./l., 100 g./l. of H 80 and 800 g./l. of H PO or 50 g./l. of caustic soda (NaOH).

Unfortunately, these reagents do not have any dissolving power on the oxide layer formed during anodization. Accordingly, even at extremely low thickness, this layer acts as a barrier layer when the voltage range allowed in industry, i.e. 48 volts at most, is used for reasons of safety. Since this layer is not porous, it does not retain the lubricants.

By contrast, other electrolytes, such as nitric acid, have an extremely aggressive effect upon the oxide layer which becomes powdery and does not have sufiicient resistance to wear or separation.

It is an object of this invention to produce and to provide a method for producing a thick, porous and resistant oxidized layer on surfaces of titanium or alloys of titanium and it is a related object to produce a surface layer of the type described which retains lubricants and which resists corrosion.

These and other objects of this invention will hereinafter appear and, for purposes of illustration but not of limitation, the specification includes a drawing in which FIGS. 1 and 2 are elevational views of strain test specimens employed for evaluation of the metals treated in accordance with the practice of this invention.

The process according to the present invention comprises carrying out anodic oxidation in an aqueous solution containing from 6% to 35% by weight of sulphuric acid and from 0.7% to 8.5% by weight of hydrochloric acid.

It is pointed out that, as a rule, hydrochloric acid has not been included in the composition of baths for anodizing titanium. The reason was undoubtedly the aggressive effects of this acid on the metal. In fact, the degreasing of titanium with chlorinated hydrocarbons has always been prohibited because it gives rise to corrosion under stress, which is attributed to the presence of traces of free hydrochloric acid in the solvent (Geduld Anodizing Titanium, Metal Finishing, April 1967).

Accordingly, it was by no means foreseeable that it would be possible to anodize titanium in an aqueous solution rich in hydrochloric acid, without any change in the mechanical properties of the metal.

The electrolysis bath used in the process according to the invention can be obtained by adding to one volume of water from 0.05 to 0.3 volumes of sulphuric acid of 66 B. and from 0.03 to 0.3 volumes of an aqueous solution of hydrochloric acid of 22 B.

In addition to these two essential acids, it is also possible optionally to add to the bath from 30 to 200 g./l. of perchloric acid or from 20 to 150 g./l. of nitric acid or, preferably, these two acids together.

Although not absolutely essential, the presence of these two acids is desirable because it improves the homogeneity of the anodization layer and increases the constancy of the voltage at the terminals, during electrolysis at constant current density.

Suitable cathode materials include titanium, zirconium or an alloy based on one or both of these two metals.

The temperature of the bath is not critical. The optimum temperature of the bath is governed by the type of metal being treated. The bath temperature is preferably maintained between 18 and 30 C. for the anodization of pure titanium and between 40 and C. for the anodization of a titanium alloy.

Steps should be taken to avoid over-rapid application of voltage at the beginning of electrolysis in order to avoid the formation of a barrier layer that would not be attacked by the electrolysis bath. The voltage at the terminals is increased at a rate which should not exceed 20 volts per minute up to its final value which, generally, amounts to between 35 and 45 volts. This level is selected to keep the current density, which can amount to between 0.5 and 2 amperes/dmfi, at a constant level.

The thickness of the oxide layer, which is governed by the quantity of current used, can be varied between 1 micron and several tens of microns. In practice, thicknesses between 2 and 15 microns are most suitable, either for protection against corrosion or for lubricant retention.

The porosity of the oxide layer varies between 20% and 80%. The pores are irregular in shape, their section varying between 2 microns and 5 microns The porous layer has the property of becoming more compact under the effect of an externally applied pressure such as arises during friction. For this reason, it insures high resistance to wear of the article being treated.

The porous oxide layer can be sealed, for example, by varnishes or resins in order to protect the metal against the corrosive etfect of external agents.

The main advantage of the porous oxide layer is its property of elfectively retaining lubricants. The tendency towards seizure, which was an obstacle to the use of titanium, is thus eliminated.

It is extremely remarkable that the anodizing treatment of this invention has hardly any effect upon the mechanical properties of the metal, notwithstanding the presence of a large quantity of hydrochloric acid in the electrolysis bath.

The following examples, given by way of illustration and not by way of limitation, relate to articles of a titanium alloy containing 6% of aluminum and 4% of vanadium, of particular importance to the aeronautical industry (alloy A), a pure titanium and a titanium alloy containing 6% of Al, 6% of V, 2% of Sn, 0.5% of Cu and 0.5% of Fe (alloy B). The object of these examples is to demonstrate the improvement which the process according to the invention provides in the resistance to seizure by virtue of the retention of a lubricant in the pores.

EXAMPLE 1 8 mm. and 2 mm. diameter bars of alloy A, in its annealed state, are anodized in an electrolytic bath containing 14% by weight of H 80 and 3.5% by weight of hydrochloric acid, obtained by adding to volumes of water, 1 volume of sulphuric acid of 66 B. and 1 volume of an aqueous solution of hydrochloric acid of 22 B.

The bath is kept at 50 C. and the counter-electrode is made of titanium. Voltage is progressively increased at a rate of 10 volts per minute until a current density of 1 ampere/dm. is obtained, corresponding to a voltage between electrodes of 40 volts. After a total of 15 minutes, the articles are washed with distilled water and dried. They are light green in color and the thickness of the porous anodic layer is 2 microns.

The treated surfaces are lubricated by the application of a calcium stearate soap containing 10% of molybdenum disulphide, after which they are subjected to cold drawing tests which are continued up to the point of seizure in the die.

The drawing ratio is calculated in accordance with the following equation:

EXAMPLE 2 8 mm. and 2 mm. diameter bars of non-alloyed titanium of commercial grade are treated in an anodizing bath obtained by adding to 10 volumes of water, 1 volume of sulphuric acid of 66 B., 1 volume of nitric acid of 40 B. and 3 volumes of an aqueous solution of hydrochloric acid of 22 B., Le. a bath containing 12.4% by weight of sulphuric acid, 2.9% by weight of nitric acid and 6% by weight of hydrochloric acid.

The bath is maintained at C. and the counter-electrode is made of titanium. Voltage is applied at a rate of 15 volts per minute until a current density of 1 ampere/ drn. is obtained, corresponding to a voltage of 48 volts. After a total of 30 minutes, the articles are washed with distilled water and dried. They are ivory in color and the 4 thickness of the anodic layer is 8 microns, with a porosity of around 30%.

The superiority of this layer to a layer obtained with conventional phosphatization and sulphuric anodization treatments, in regard to deformation by drawing, was demonstrated as in the case of Example 1. It was possible to obtain a total deformation of 200% for the 8 mm. di ameter bars and 400% for the 2 mm. diameter bars.

By way of comparison, for 2 mm. diameter bars, a high quality conversion layer obtained by phosphatization only allows 200% deformation, while a layer anodized in a conventional sulphuric medium only allows a deformation of 70%.

EXAMPLE 3 A polished, 14 mm. diameter cylinder of alloy A was anodized in an electrolytic bath obtained by adding to 10 volumes of water 1.5 volumes of sulphuric acid of 66 B.

1 volume of a solution of hydrochloric acid of 22 B 1 volume of nitric acid of 40 B.

1 volume of perchloric acid of 56 B.

The bath is maintained at 50 C. The voltage between electrodes is increased progressively at a rate of 15 volts per minute up to 40 volts which produces a current density of 1 ampere/dm.

After 30 minutes treatment, an oxide layer is obtained which has a thickness of 10 microns and a porosity of 70%.

After washing and drying, the porous layer is impregnated with lubricant, as in Example 1.

By way of comparison, 3 other identical cylinders were prepared:

One was treated by anodization in a 10% sulphuric acid solution at a temperature of 20 C. and a voltage of 40 volts,

Another was covered with a layer formed by chemical phosphatization,

The third was coated by spraying to provide a layer of molybdenum 15 microns thick.

Friction tests on steel were carried out under the following conditions: a shoe of steel with a Rockwell C hardness f0 60 is applied to the cylinder of alloy A turning at 130 r.p.m. The loads applied to the shoe are calculated in such a way that the maximum stress at the shaft-shoe interface is a specific fraction (20%30%-40%50%) of the elastic limit of the alloy, which amounts to 84 kg./ mm.

Calculation of the stress a is determined by Hertz formula l..! l 1 am....0.42 LR with where The results obtained are set out in Table I.

TABLE I Number of revolutions of the cylinder before seizure Treated by the Maximum stram process Anodized Treated Sprayed expressed in percent according in sulby phoswith of the elastic limit; to the phuric phatizmolybof alloy A invention medium ation denum 20% (16.8 kg./mm. 110, 000 #1, 000 70, 000 30% (25.2 kg./mJu. 102, 000 100 #500 100, 000 40% (33.6 k ./rnm. 160, 000 50 100 145, 000 60% (42 kg. rum?) 130, 000 50 100 102, 000

It can be seen that the anodic layer obtained by the process according to the invention is markedly superior to that obtained by anodization in a sulphuric bath or by phosphatization. Its behavior in the presence of dry lubricant compares favorably with the spraying of molybdenum which, of course, is a much more expensive material.

In addition, with the layer formed by the process of this invention, friction measured during the test in this example is extremely low. From 0.05 at the beginning of the test, it increases to 0.01-0.02 after several thousand revolutions and remains at this level up to seizure.

EXAMPLE 4 A 14 mm. diameter polished cylinder of alloy A Was andoized in a bath obtained by adding to volumes of water 0.5 volumes of sulphuric acid of 66 B. and 0.3 volumes of an aqueous solution of hydrochloric aicd of 22 B.

The bath is maintained at 40 C. The voltage between electrodes is progressively increased at a rate of 15 volts per minute up to 30 volts which produces a current density of l ampere/dmfi.

After 10 minutes treatment, a bright green oxide layer, having a thickness of 1 micron and a porosity of around is obtained.

This layer 'behaves satisfactorily in the friction tests with a life of 70,000 revolutions, under a strain equal to of the elastic limit.

EXAMPLE 5 A 14 mm. diameter polished cylinder of alloy B was anodized in an electrolytic bath obtained by adding to 10 volumes of water 1 volume of sulphuric acid of 66 B. and 0.5 volumes of an aqueous solution of hydrochloric acid of 22 B.

The bath is maintained at 70 C. Voltage is applied at a rate of 15 volts per minute until a current density of l ampere/dm. under 56 volts is obtained.

After 20 minutes treatment, a dark grey layer, having a thickness of 6 microns, and a porosity of around 60%, is obtained.

This layer behaves in the same way as the layer formed on alloy A, when subjected to the friction test under the conditions of Example 4.

Two types of test specimens, illustrated in FIGS. 1 and 2, were used to measure the possible influence of the anodizing treatment upon the mechanical properties of alloy A. The first, which is a notched cylindrical bar, is used to measure embrittlement under tension (Pratt and Whitney test). The second (FIG. 2), whose effective part is frustoconical, is used to measure fatigue by rotational flexure.

In the case of the bars of alloy A in its annealed state, 10 identical test specimens of each type, 5 of which were retained as control and 5 anodized by the process according to the invention, were turned under the same conditions as the cylinders of Example 2. They were then washed and dried.

The notched test specimens were subject to an initial strain of 116 kg./mm. which was increased by 5 kg./ mm. every five hours.

All the test specimens failed between and 50 hours under strains of 156 kg./n1m. to 161 kg./mm. without any possibility of distinguishing between the anodized and nonanodized test specimens.

The tests carried out on the test specimens shown in FIG. 2 demonstrated that there was only a minimal difference (of the order of 5%) between the fatigue limits (at 6 3x10 alternations) of the non-anodized (control) and anodized test specimens.

Accordingly, the mechanical properties of the metal are shown to be substantially unaffected by the anodizing treatment according to the invention.

We claim:

1. A method for obtaining titanium or titanium alloy parts provided with a low friction coefiicient surface and capable of resisting gripping or seizure by rubbing under high pressure which consists of forming on the surface of the parts a porous and resistant oxide layer having a thickness of more than 1 micron and an open porosity ranging between 20% and with dimensions of pore sections ranging between 2 and 5 square microns, said oxide layer being formed by anodization under DC in an aqueous solution which contains a quantity of sulfuric acid ranging between 6% and 35% by Weight and a quantity of hydrochloric acid ranging between 0.7% and 8.5% by weight.

2. A process as claimed in Claim 1 in which the bath contains in addition an acid selected from the group consisting of 30 to 200 g./ 1. of perchloric acid and 20 to g./ l. of nitric acid and mixtures thereof.

3. A process as claimed in Claim 1 in which the electrolysis is carried out with direct current, using as the cathode a metal selected from the group consisting of titanium, zirconium and an alloy based upon titanium and/ or zirconium.

4. A process as claimed in Claim 3 in which the electrolysis is carried out under a voltage of less than 48 volts established by progressive increase in the voltage at a rate which does not exceed 20 volts per minute.

5. A process as claimed in Claim 1 in which the temperature of the bath is within the range of 18 to 80 C.

6. A process as claimed in Claim 5 in which the temperature does not exceed 30 C. for anodization of pure titanium.

7. A process as claimed in Claim 5 in which the temperature is within the range of 40 to 80 C. for the anodization of titanium alloys.

8. An article having a surface of titanium or titanium alloy in which the surface has an anodic layer of a porous lubricant retaining resistant layer of oxide having a thickness of at least 1 micron and an open porosity within the range of 20% to 80% of the volume with a pore size within the range of 2 microns to 5 microns and which is formed by positioning the article as an anode for direct current anodization in a solution containing as essential ingredients 635% by weight H 80 and 0.7-8.5% by weight HCl and which contains as non-essential ingredients 30-200 grams per liter perchloric acid and 20-150 grams per liter nitric acid, and mixtures thereof, the remainder being water.

9. An article as claimed in Claim 8 in which a lubricant is present in the porous anodic layer.

10. An article as claimed in Claim 8 in which a resinous material is present in the porous anodic layer to seal the layer and avoid corrosion from external agents.

References Cited UNITED STATES PATENTS 2,949,411 8/1960 Beck 20456 R 2,647,079 7/1953 Burnham 20438 E FOREIGN PATENTS 1,046,929 10/1966 Great Britain 20456 R JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R.

20438A, E, 56 R 

1. A METHOD FOR OBTAINING TITANIUM OR TITANIUM ALLOY PARTS PROVIDED WITH A LOW FRICTION COEFFICIENT SURFACE AND CAPABLE OF RESISTING GRIPPING OR SEIZURE BY RUBBING UNDER HIGH PRESSURE WHICH CONSISTS OF FORMING ON THE SURFACE OF THE PARTS AV POROUS AND RESISTANT OXIDE LAYER HAVING A THICKNSSS OF MORE THAN 1 MICRON AND AN OPEN POROSITY RANGING BETWEEN 20% AND 80% WITH DIMENSIONS OF PORE SECTIONS RANGING BETWEEN 2 AND 5 SQUARE MICRONS, SAID OXIDE LAYER BEING FORMED BY ANODIZATION UNDER DC IN AN AQUEOUS SOLUTION WHICH CONTAINS A QUANTITY OF SULFURIC ACID RANGING BETWEEN 6% AND 35% BY WEIGHT AND A QUANTITY OF HYDROCHLORIC ACID RANGING BETWEEN 0.7% AND 8.5% BY WEIGHT. 